316 Prof. Bache's Lecture on the Gulf Stream. & 
graduated by trial. The brass and silver portions receive 2 © 
thick coating of gold by the electrotype process to prevent the 
action of the sea-water upon them. 
When kept clean by frequent washing in fresh water, and in 
i Jards to guard 
good order and frequently compared with the 
against accidental derangements, these thermometers answer ad- © 
mirably all the required conditions. The length of the coil 
measured along its axis should not be less than six inches, as 
the interposition of wheels to magnify the motion, should as far 
as possible, be avoided. The water being all around the coil, 
which is a good conductor, and has a low specific heat, the instru- 
ment readily feels the temperature of the part of the sea where 
it is exposed, and registers it to less than half a degree (say 
0-2) with certainty. The box which covers the coil and indica- 
ting part of the thermometer is merely to protect it from acel- 
dental injury, and is open so as to permit the sea-water to 
pass freely through it. Plate IV gives a view of Saxton’s 
metallic thermometer, and of its various parts in detail. Al 
though there seemed no reason to doubt that this instrument 
was free from any effects of pressure, it was deemed desirable to 
actually try it by extreme pressure and a series of experiments 
made by J. M. Batchelder, Esq., showed that at pressures less than 
that corresponding to 600 fathoms, the effect was less than one 
degree (0°25 to 1°) and at pressures from 600 to 1500 fathoms 
the change amounted to little more than from 7° to 9° Fahr., the 
index returning when the pressure was removed. For great 
depths the effects of pressure must be ascertained, as it is specific 
in each instrument and probably depends chiefly upon some 
mechanical defect in the construction, perhaps in the soldering.* 
sure, was a very ingenious one for testing hydraulic engines DY 
Mr. Thomas Davison of the Novelty Iron Works of New York. 
Z 
} 
2 
i 
‘= 
te 
For Depths—Where the depth becomes considerable the — 
usual sounding line fails entirely to give it, especially if there 18 
a current and more especially if there is besides, a counter-cul” 
rent. The amount of “stray line” is very variable. 
ee, Lieuts. Berryman, Brooke and others of our navy; 
and by Commander Dayman and others of the British navy; 
and especially by Prof. Trowbridge of the Coast Survey 1» his 
memoir read before the Association (“Deep Sea Sounaing®, 
Jeet ie been ably examined of late years by Commanders Maury. 
and §. 
by W. P. Trowbridge, Assistant U. S. Coast Survey,) at the 
g ssistan oas vey J ; 
meeting in Baltimore and re-published in the American 70 
of Science and Arts, vol. xxviii for the year 1858. 
* Gulf Stream Explorations ; Third Memoir Proceedings Amer. Assoc. Adv. S¢, 
13th Meeting, Springfield, 1859, and this Jour., [2], vol. xxix, 1860. 
al 
